1. Field of the Invention
This invention relates to an image forming apparatus which is capable of forming images on both sides of a transfer member.
2. Description of the Related Art
There has been known apparatuses of the kind arranged to be capable of forming images on both sides of a transfer member. An example of them is arranged as shown in FIG. 1 of the accompanying drawings. A first toner image which is formed on a photosensitive member drum 42 is transferred at a transfer station T to the first side of a transfer member P which is supplied from a paper supply cassette 41. Following this, the transfer member P is separated from the photosensitive member drum 42. The first toner image is then fixed to the transfer member P by a fixing device 43. The transfer member P is supplied via a reversing device 44 to a paper feed tray 45 to be stowed thereon. Next, the transfer member P is supplied from the paper feed tray 45 to the drum 42 with the second side of the transfer member P facing to the drum surface. A second toner image which is newly formed on the drum 42 is then transferred at the transfer station T to the second side of the transfer member P. The fixing device 43 fixes the second toner image to the transfer member P and moves the transfer member P to the outside.
However, the above stated conventional arrangement has presented the following problem. In copying the images on both sides of the transfer member P, an image is first transferred and fixed to the first side of the transfer member P. Following that process, the transfer member P is temporarily stowed on the paper feed tray 45 before copying another image on the second side of the transfer member P. This arrangement requires two periods of time for copying the images on the upper and reverse sides of the transfer member P respectively. Therefore, even in the case of an apparatus which is capable of speedily accomplishing single side copying, the conventional arrangement has necessitated a more than two times longer period of time for dual-side copying than single-side copying. This has caused a drop in a reproducible number of sheets, or copying speed, per unit time.
In connection with this problem, U.S. Pat. Nos. 3,536,398 and No. 3,580,670 have disclosed image forming means of the kind having two sets of image forming means which are arranged to include fixing devices respectively and to continuously transfer and fix images onto both sides of a transfer member one after another. However, the use of a plurality of fixing devices presents another problem as it results in an increase in the power consumption and also in an increase in size of the apparatus. Meanwhile, an apparatus of another kind has been disclosed in U.S. Pat. Nos. 3,775,102. That apparatus uses only one fixing device and is arranged to transfer images to first and second sides of a transfer member one after another and then to fix the images to the first and second sides. In that case, however, the image on the first side tends to be disturbed in transferring another image to the second side.